A **geometric sequence ** is a sequence of numbers in which the ratio between consecutive terms is constant.

We can write a formula for the *n*^{th} term of a geometric sequence in the form

*a*_{n} = *cr*^{n},

where *r* is the common ratio between successive terms.

**Example 1:**

{2, 6, 18, 54, 162, 486, 1458, ...}

is a geometric sequence where each term is 3 times the previous term.

A formula for the *n*^{th} term of the sequence is

**Example 2:**

is a geometric series where each term is –1/2 times the previous term.

A formula for the *n*^{th} term of this sequence is

**Example 3:**

{1, 2, 6, 24, 120, 720, 5040, ...}

is **not** a geometric sequence. The first ratio is 2/1 = 2, but the second ratio is 6/2 = 3.

No formula of the form

*a*_{n} = *cr*^{n} can be written for this sequence.

See also arithmetic sequences.